Elastic motors have been employed for centuries and have found particular application in model airplanes for over 150 years. Typical prior art elastic motors, such as those used in a model airplane, generally comprise a rubber loop threaded through a hook on a propeller shaft and is further attached to another hook at the tail of the craft. As the motor is wound up it first twists the elastic into a skein, then a row of knots form and spread along the whole length. A third stage occurs when a row of knots forms in the already knotted skein. When this row is complete the rubber is substantially stretched to its limit.
Upon release of propeller, there is a burst of power. When this is spent, a period of slowly declining torque follows for the majority of the motor run and is followed by a decline to zero torque.
The torque characteristics of the prior art elastic motor devices, like those described above, make model airplane design a considerable challenge. Clearly, this is because it is difficult for a designer to properly construct design constraints when the source of propulsion has such wavering torque characteristics. Aside from model airplane design, conventional elastic motors have undesirable performance in other applications as well. The knotting of the rubber introduces internal friction in the wound skein that can be eased somewhat by lubrication. Lubrication, however, drastically reduces the useful life of the rubber.
Therefore, the shortcomings of the prior art suggest a strong need for an elastic motor design that has a constant torque characteristic and does not damage the elastic material.
One particular invention which aims to answer this need for a constant torque producing elastic motors is disclosed in McAneny U.S. Pat. No. 4,629,438 which describes a rubber band powered motor for a model airplane. McAneny teaches a method of providing an extended flight by producing a more stable torque. The elastomeric members are either fully stretched, fully relaxed, in brief stretching mode or in brief relaxing mode. As a result, McAneny relies on tapes and gears, coupled to multiple elastomeric members, to absorb the sudden bursts and function to produce a more stable torque output. Additionally, McAneny relies upon components, namely gears and tapes, which may add significant weight, cost and size to the design, all of which are undesirable characteristics for most applications.
Thus, there exists a need for an elastic motor which can provide an approximately constant torque output while still retaining desirable and practical characteristics such as light weight, low cost and small size.